1. Field of the Invention
The present invention relates to an image reading method and apparatus, and more particularly, it relates to an image reading apparatus and an image reading method including an automatic document feeder of the sheet-through reading type for sequentially transporting original documents to a document reading position and reading the images of the original documents in an automatic manner.
2. Description of the Related Art
In known image reading apparatuses provided with an automatic document feeder (ADF=Auto Document Feeder) of the sheet-through reading type, when an image of an original document put on a document glass is to be read, a scanning carriage is moved to carry out the reading of the original document image. When the image of the original document is being read out while the original document being transported by means of the ADF, the scanning carriage is moved to an ADF document reading position where the image of the document is read while the scanning carriage is stopped there. In case where an image of an original document is read out in this manner, shading processing is carried out to read a white color that becomes a reference for deciding shades of the image to be read, and after such shading processing is done, the image of the original document is being read while the document is being transported along a reading glass which is arranged at the ADF document reading position for exclusive use for ADF document reading.
With the image reading apparatus as described above in which the image of the original document is being read out (or through read) while the document is being transported along the reading glass, when a speck of dirt (e.g., even dirt just like a point or spot) adheres to the reading glass exclusively used for reading ADF original documents, it appears as an abnormal line extending in a sub scanning direction on the image thus read. If such a line appears upon image formation, there might be the case where the image formed becomes not only unsightly but also unusable for printed matter.
Therefore, when such an abnormal line (e.g., a so-called black stripe) due to dirt appears on the readout image, the user has to clean the reading glass so as to obtain a good quality image, which, however, is troublesome and annoying. In addition, in Japanese patent application laid-open No. Hei 11-109521, a special cleaning mechanism is mounted on an image reading apparatus for detecting and removing dust, stain or the like on a mirror based on an image signal, but in this case, there is a problem of bringing about a lot of cost increase.